Split exhaust manifolds with pulse separation can be used to deliver higher exhaust pressure to the turbocharger resulting in improved turbocharged engine performance. Separated exhaust paths also prevent the trapping of exhaust gases in adjacent cylinders which may cause misfiring of the cylinder. Diverse approaches may be used to control exhaust manifold pressure and turbine speed at varying operating conditions to regulate boost.
One example approach is shown by Danet et al. in WO 2008/078020 wherein the boosted engine system includes a variable volume exhaust manifold. Therein, exhaust flow is directed via two separate paths to the twin scrolls of the turbocharger to provide boost and engine speed control. The split exhaust pipes are each connected to a storage volume upstream of the turbine and each volume is controlled by a butterfly valve that opens or closes said storage volume based on engine speed. At low engine speed, the storage volumes are closed to preserve exhaust pressure but at high engine speeds, the storage volumes are opened to increase exhaust volume and reduce pumping losses. The valve can also assume a half-closed or half-open position.
The inventors herein have identified potential issues with the above approach. Even though split manifolds offer pulse preservation, depending on the engine speed they may expose exhaust valves to higher pressures under some conditions. For example, even at lower speeds, exhaust pressure pulsations in the split manifold can be sufficiently high to force open the exhaust valves at untimely moments in the combustion cycle. For example, exhaust pressure pulses can generate peak exhaust pressures in the split manifold that are sufficiently high to force open an exhaust valve during an intake stroke of the cylinder, inadvertently admitting exhaust gases into the cylinder. This can have negative consequences, including significant loss of engine power and efficiency, while increasing combustion instability.
The inventors herein have recognized the above issues and identified various approaches to address them.
One approach provides a method, including opening a volume control valve on a split exhaust manifold of an engine responsive to engine operating conditions, but closing the volume responsive to an operator tip-in. For example, even though engine speed conditions may dictate an open valve for desired steady-state engine efficiency, the increased responsiveness of turbocharger spool-up obtained with a closed valve position may provide desired operator performance. Thus, by at least temporarily closing the valve during such transient conditions, improved performance can be achieved. Once through the transient (e.g., after the boost level has reached a threshold), the valve can be returned to its desired position based on engine speed.
Likewise, even though engine speed conditions may dictate a closed valve for desired steady-state engine efficiency, the increased peak exhaust backpressure may cause an exhaust valve of one of the cylinders communicating with the exhaust to inadvertently open during an intake stroke. Thus, by at least temporarily opening the volume control valve during such conditions, degraded combustion performance can be reduced. Once peak exhaust back-pressures recede, the valve can be returned to its desired position based on engine speed in order to preserve exhaust pulses for efficiently driving the turbocharger. In this way, it is possible to balance a system efficiently while reducing inadvertent opening of exhaust valves.
In another embodiment, a method includes, during a transient increase in torque demand (e.g., a pedal tip-in), where the torque request increases beyond a threshold and a boost pressure above a threshold is commanded, the volume control valve may be closed independent of other engine conditions (e.g., higher exhaust pressure causing untimely opening of exhaust valves) to provide increased pressure exhaust pulses to the turbocharger. On the other hand, if boost pressure is already sufficiently high and the higher exhaust pressure is present, the volume control valve can be opened to decrease exhaust peak exhaust pressure and reduce the inadvertent opening of the exhaust valves.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.